Typical internal combustion engine cooling systems include a water pump driven by a belt for circulating coolant through an engine block and a radiator. The pump is directly driven by the engine such that the rotational speed of the pump is directly proportional to that of the engine. Furthermore, the pump is driven continuously as long as the engine is operating. As such, coolant is circulated at all times including engine start up when the temperature of the engine may be less than a desired operating temperature. Prior to reaching the desired operating temperature, the engine may output increased undesirable emissions. Circulating cooling water immediately after engine start up may increase the time required for the engine to reach the desired operating temperature. Consequently, the quantity and duration of emissions production is greater than optimal. Furthermore, because the engine is operating for a longer period of time at a temperature less than the desired operating temperature, a cabin heating system may also require increased time to pump warm air toward the vehicle occupants.
Some automobiles have been equipped with magneto-rheological clutches to variably control the water pump regardless of engine operating speed. Unfortunately, these pump control systems are relatively heavy, complex and expensive. Accordingly, it may be desirable to provide a simplified, low-cost on/off water pump assembly.